Conventionally, metal oxides such as antimony-doped tin oxide (ATO) and tin-doped indium oxide (ITO) are used as a transparent conductive film for an electrode of display element such as liquid crystal display element and electroluminescence display element, and a heating resistor in a window glass of an automobile, an aircraft or a building for anti-fogging or anti-icing purpose.
In recent years, it has been found that oxide semiconductors such as ZnO, In2O3 and In—Ga—Zn—O as one type of the metal oxides are semiconductors that exhibit high carrier mobility compared to an amorphous silicon, and development of a field-effect transistor (FET: Field-Effect Transistor) in which these oxide semiconductors are used in its active layer has been active.
As a method for forming a thin layer of such metal oxides, a vacuum deposition method and a sputtering method are common. For example, a semiconductor thin layer which employs a vacuum deposition technology such as sputtering and which includes indium, positive divalent element (especially, zinc, magnesium, copper, cobalt, nickel, calcium) and oxygen and has a specific resistance of 10−1 Ωcm to 108 Ωcm is proposed (see PTL1).
However, there is a problem that a complex and expensive apparatus is required for implementing these methods. There is also a problem that it is difficult to form a thin layer with a large area.
Thus, as a method which is easier and enables a large area, a coating solution that an inorganic metal compound or an organic metal compound is dissolved in an organic solvent and that another metal is added as an activator to impart higher electrical conductivity, and a coating method which uses the coating solution have been studied.
For example, for the purpose of forming a thin layer having a high electrical conductivity and transmittance, a composition for forming a transparent conductive film including an organic compound which may be coordinated to an inorganic indium compound, magnesium compound and indium is proposed (see PTL2). Also, a composition for forming a transparent conductive film that indium nitrate, a condensate of a polyhydric alcohol and an activator is dissolved in an organic solvent is proposed (see PTL3).
However, these proposed technologies are technologies of a composition for forming a transparent conductive film, and a sufficient functionality as an active layer of a field-effect transistor cannot be obtained with the obtained transparent conductive film, and there is a problem that available applications are limited.
Also, a metal oxide precursor solution that an inorganic metal salt as a metal oxide precursor is dissolved in water or ethanol as a solvent and a method for preparing an oxide semiconductor by coating the metal oxide precursor solution on a substrate are proposed (see PTL4). With this proposed technology, the oxide semiconductor is considered as an active layer of a field-effect transistor.
However, with this proposed technology, the coating solution spreads thinly when the metal oxide precursor solution is coated on the substrate, and there is a problem of low precision of a shape of the obtained oxide semiconductor.
Thus, there currently is being asked to provide a coating liquid for forming a metal oxide thin film which enables to produce a metal oxide thin film having a desired volume resistivity easily and in a large area and which has a high precision for forming a metal oxide of a desired shape.